Why is the Sky Blue?
Strand: Energy and Control
Grade: 4
Topic: Light and Sound Energy
Specific Expectation: identify, through observation, colour as a property of light (e.g., use
prisms to show that white light can be separated into colours)
Materials:
∞ Large glass vase
∞ Strong flashlight
∞ 32 ounces water
∞ 1mL whole milk
∞ Long handled spoon
∞ One piece of white cardboard
Procedure:
1. Fill the glass vase with 4 cups of water.
2. Shine flashlight onto white cardboard and note that the beam shows a bright white to
yellow light.
3. Now shine the flashlight through the water in the vase onto the cardboard. You should
notice that the light is still the bright white/yellowish light.
4. Now, add 1mL of whole milk to the water and stir well with the spoon.
5. Shine the flashlight through the milky water onto the piece of cardboard. You should
immediately notice that the light is a pinkish orange and that the milky water appears to
be a light blue.
What Happened?
The only electromagnetic waves we can see are visible light, which we see as the colours of
the rainbow. Each colour has a different wavelength. Violet has the shortest wavelength and red
has the longest wavelength. When all the waves are observed together,
they create white light, which is why the light from the flashlight beam
shows white light. If you were to shine white light through a prism, it
would break up into its separate colour wavelengths.
No diffraction occurs when the white light of the flashlight is shone
through the water in the vase. The light continues to be observed as white light. However, when
the light is shone through the milky water, it breaks up into blue light (the shortest wavelength in
the visual spectrum) and red light (the longest wavelength in the visual spectrum).
The particles of fat in the milk are small and fairly uniform in size. When the light hits the
fat particles, they scatter the light rays. The blue wavelengths of light are more easily scattered
by the fat particles than the red wavelengths. The blue light is scattered in many
directions making the milky water to appear blue in colour. The red light does
not scatter and penetrates the milky water to show up on the white cardboard.
Sunlight reaches Earth's atmosphere and is scattered in all directions by all
the gases and particles in the air. Blue light is scattered in all directions by the
tiny molecules of air in Earth's atmosphere just as the fat particles scatter the blue light in the
milky water. Blue is scattered more than other colors because it travels as shorter, smaller waves.
This is why we see a blue sky most of the time. This effect is called Raleigh Scattering.
As the Sun gets lower in the sky, its light is passing through more of the atmosphere to reach
you. When the air is clear the sunset will appear yellow, because the
light from the sun has passed a long distance through air and some of
the blue light has been scattered away. Even more of the blue light is
scattered, allowing the reds and yellows to pass straight through to your
eyes. Sunsets over the sea may also be orange, due to salt particles in
the air, which are effective Tyndall scatters. The sky around the sun is
seen reddened, as well as the light coming directly from the sun. This is because all light is
scattered relatively well through small angles-but blue light is then more likely to be scattered
twice or more over the greater distances, leaving the yellow, red, and orange colours.
References:
Spectrum, (2007). The Electromagnet Spectrum. NASA. Retrieved from:
http://science.hq.nasa.gov/kids/imagers/ems/index.html
Gibbs, Philip. (1997). Why is the sky blue? Retrieved from:
http://math.ucr.edu/home/baez/physics/General/BlueSky/blue_sky.html
Simple, (1997). Science Made Simple. Retrieved from:
http://www.sciencemadesimple.com/sky_blue.html
Space, (2005). Space Place. NASA. Retrieved from:
http://spaceplace.nasa.gov/en/kids/misrsky/misr_sky.shtml
Opportunities and Other Considerations:
This activity can be used to determine whether the source of light (natural or artificial) can be
diffracted into the same light spectrum. This activity can be modified by using a flashlight and a
compact disc to show how light can be broke into its basic parts.
Glossary:
Electromagnetic wave: Light, microwaves, x-rays, and TV and radio transmissions are all kinds of
electromagnetic waves. They are all the same kind of wavy disturbance that repeats itself over a
distance called the wavelength.
Wavelength: The wavelength is the distance between repeating units of a wave pattern.
Atmosphere: Atmosphere is the general name for a layer of gases that may surround a material body
of sufficient mass.
Diffraction: Refers to the bending of light waves when they interact with obstacles or molecules in
their path.
Name of Presenters: Christy Genco and Lindsey Steingard